1. Field of the Invention
The present invention relates to a liquid-crystal electro-optical device of a birifringence mode using a ferroelectric or antiferroelectric liquid-crystal material.
2. Description of the Related Art
Recently, attention is paid to liquid-crystal display devices (LCD). Among them, in particular, the liquid-crystal display devices of the twisted nematic (hereinafter referred to as "TN") type using nematic liquid crystal and the super twisted nematic (hereinafter referred to as "STN") type have been widely known and practically used.
Also, the liquid-crystal display devices of the active matrix type using the nematic liquid-crystal material in which a switching element such as a thin-film transistor (TFT) is provided for each pixel have been extensively developed as the device capable of performing high speed, high-contrast and multi-gradation display.
The fundamental structure of the liquid-crystal electro-optical device of the TN type and the STN type will be now described. An orientation film is coated on a substrate having an electrode, sintered and then subjected to rubbing as an orienting process to form a first substrate. Likewise, an orientation film is coated on a substrate having an electrode, sintered and subjected to rubbing to form a second substrate. The first and second substrates are provided so that the respective electrodes are opposed to each other, and liquid crystal is held between these substrates.
On contact surfaces between the respective substrates and a liquid-crystal layer, the liquid-crystal is aligned in a rubbing direction in accordance with a regulating force of rubbing. The rubbing directions of the upper and lower substrates are shifted from each other by 90.degree. in the case of the TN-type device and by 200.degree. to 290.degree. in the case of the STN-type device. The liquid-crystal molecules in the vicinity of the middle of the liquid-crystal layer are aligned in the spiral form so that energy becomes smallest between the liquid-crystal molecules positioned at 90.degree. to 290.degree..
These liquid-crystal molecules arrayed in the spiral form are rearranged in parallel with or perpendicular to a direction of an electric field due to the dielectric anisotropy of the liquid-crystal molecules by applying voltage across the liquid crystal layer, thereby to loose the spiral structure thereof. The transmitted light amount is changed in such a manner that, when the liquid-crystal molecules are perpendicular to the surface of the substrate, it exhibits a light state, whereas when the liquid-crystal molecules are in parallel therewith, it exhibits a dark state. Also, the state of such liquid-crystal molecules is continuously changed in accordance with the applied voltage, and the transmitted light amount is changed in accordance with the change of the state of the molecules. Therefore, a gradation between the bright (transmitted) state and the dark (non-transmitted) state, that is, a halftone is obtained by properly controlling applied voltage.
Obtaining the halftone is very effective in coloring, and the device can cope with full-coloring if a response speed of the liquid crystal is satisfactory.
The nematic liquid crystal has a low response speed of 100 msec, and therefore does not provide a sufficient characteristic for display of an animation or the like which requests a high-speed response.
Also, because the nematic liquid crystal has the fluidity, if the device is used in a standing state, the liquid crystal is gathered in a lower portion of the device, whereby the lower portion of a cell has such a shape as to be swelled. As a result, since the thickness of the cell is largely changed within the device, coloring or a color shade is produced, or the response of the liquid crystal is not uniform even though the same voltage is applied thereto.
Recently, because it is desired that the screen of the device is made large and the response speed thereof is made high, the above-mentioned problems of the nematic liquid crystal comes more serious.
On the other hand, the LCD using the ferroelectric liquid crystal has also been developed. The ferroelectric liquid crystal can perform switching operation at high speed of several tens .mu.sec because the liquid-crystal molecules have spontaneous polarization. The ferroelectric liquid crystal or the antiferroelectric liquid crystal has spontaneous polarization, thereby to enable high-speed operation at such a response speed of several to several hundreds .mu.sec. Thus, it responds at high speed more than about three digits.
In the liquid-crystal electro-optical device using the ferroelectric liquid crystal or the antiferroelectric liquid crystal, the liquid-crystal molecules can be oriented in accordance with a regulating force of orientation if at least one of substrates is subjected to an orientating process. These liquid-crystal molecules have a laminar structure in which they are regularly piled on each other from one surface of the substrate toward the other surface thereof. Also, they have a laminar structure in a direction parallel to the substrate.
Because of such layer structures, the ferroelectric liquid crystal or antiferroelectric liquid crystal is poor in fluidity, and in the case where the device is in a standing state, there is advantageous in that the thickness of the cell is kept constant, thereby to enable uniform display without liquid crystal being gathered in the lower portion of the device like in the case of a nematic liquid crystal.
Naturally, in an SmC* phase exhibited by the ferroelectric liquid crystal material, liquid crystals are oriented so that the long axis of the liquid-crystal molecules is inclined by a certain inclined angle with respect to a normal (almost parallel to the substrate) of a layer provided in the liquid-crystal material, and this forms a spiral structure in which the liquid-crystal molecules have the direction of orientation vector which is twisted from one layer toward another layer in a bulk state, and because the spontaneous polarization having the liquid-crystal molecules is offset as a whole, ferroelectricity cannot appear.
Therefore, there has been proposed a liquid-crystal electro-optical device of the so-called surface stabilizing type exhibiting ferroelectricity by Clerk. In the fundamental structure, the ferroelectric smectic liquid-crystal material is held between a pair of substrates having the electrodes, and the liquid-crystal molecules are arranged in parallel with the substrate and optically uniaxially oriented so that a layer having a liquid-crystal material is formed perpendicular to the substrate or inclined with respect to the substrate. At this time, an interval between the pair of substrates is set to about 1 .mu.m so that the spiral structure taken by the liquid-crystal material in the bulk state is loosened. Further, as a result of loosing the spiral structure, the directions of the orienting vectors taken by the liquid-crystal molecules are in two stable orienting states, that is, bistable orienting states.
With the above-mentioned structure, the polarity of an electric field applied to a pixel electrode is inverted so that a high-speed response can be made between the above-mentioned two states by torque resulting from a product of spontaneous polarization possessed by the liquid crystal and the electric field applied by the electrodes.
The orientation of the spontaneous polarization possessed by the liquid-crystal molecules per se is changed by 180.degree. (hereinafter referred to as "inversion") by applying voltage between the substrates. The liquid-crystal molecules has an orientation which is changed by a certain angle with respect to the optically uniaxially orienting direction, and the orientation of the liquid-crystal molecules is inverted by applying voltage, thereby to perform switching operation from a bright (transmission) state to a dark (non-transmission) state, or from the dark state to the bright state.
Since the liquid-crystal electro-optical device using nematic, ferroelectric or antiferroelectric liquid crystal utilize the optical anisotropy of the liquid molecules, it has polarizing plates outside of both the substrates to obtain an electro-optical characteristic.
In the case of using the ferroelectric or antiferroelectric liquid-crystal material, an optical axis of one polarizing plate is matched with a direction in which one orienting state is exhibited in either of two stable states, and the other polarizing plate is disposed so as to be optically axially perpendicular to the one polarizing plate.
As a method of uniaxially orienting the liquid-crystal molecules, there has been known a method of forming a means for providing an orienting regulating force, which makes the liquid-crystal material uniaxially oriented (hereinafter referred to as "uniaxial orientation means"), on a surface which is in contact with the liquid-crystal material of the substrates, between which the liquid crystal is held. The rubbing method has been typically known. The rubbing method is a method in which an orientation film having a thickness of 100 to 500 .ANG., which is usually made of organic macro-molecules or the like, is formed on a surface having the electrode of the substrate, and the surface of the orientation film is rubbed with cloth or the like in one direction (rubbing process), thereby to provide an optically uniaxial orienting regulating force which allows the liquid-crystal molecules to be arranged on the orientation film in one direction. The surface of the substrate or the electrode may be directly subjected to the rubbing process. The rubbing method is widely used in the TN-type or STN-type liquid crystal electro-optical device using the nematic liquid crystal, and also most generally used in the ferroelectric liquid crystal as an excellent orienting method which is simple and easy in making the area of the liquid crystal large.
Since the ferroelectric liquid crystal has a high orderliness and a layer structure, once the liquid crystal is oriented, the orientation thereof is not disordered as far as the layer structure is not destroyed. Therefore, it is not limited to the orientation using the rubbing method, and even in the orienting method using no orientation film, that is, performing only an initial orientation, such as a shearing method, a magnetic orienting method, a temperature gradation method or the like, the liquid-crystal molecules are sufficiently oriented, thereby to enable switching operation. However, these methods are used experimentally, however, because they require much time for orienting the liquid-crystal material and are improper for manufacturing of a large-area device so as to be not practical, they are not much used industrially.
Also, as another method, there is a rhombic vapor deposition method in which SiO or the like is obliquely vapor-deposited with respect to the surface of the substrate, however, there is a problem in productivity. Further, when the rhombic vapor deposition method is applied to the large-area substrate, there arises such a problem that differences in vapor-deposition angle, vapor-deposition orientation or the like between the respective points on the substrate cannot be ignored. Therefore, in the current ferroelectric liquid-crystal electro-optical device, the rubbing method is used as the orienting method which is widely industrially used.
Further, because the ferroelectric liquid crystal or the ferroelectric liquid crystal can be switched at a higher speed by about three digits in comparison with the nematic liquid crystal, an on-state and an off-state are controlled every display frame to perform gradation display in accordance with a display time, that is, to enable so-called frame gradation display. A digital gradation display with multi-gradations can be performed by controlling the on/off period in the form of a digital value by using TFTs. The details are disclosed in Japanese Patent Unexamined Publication No. 6-102486 published Apr. 15, 1994 by Konuma et al.
In this case, an amorphous silicon TFT may be used for the TFT. However, for the purpose of allowing the TFT to cope with the high-speed switching operation of the ferroelectric liquid crystal and of obtaining a higher speed, a multi-gradation and a high contrast ratio in the digital gradation display, injection of charges into a pixel is necessary to perform more smoothly. For this reason, there is used a crystalline silicon TFT which is operated at higher speed than the amorphous silicone TFT by about four digits and capable of allowing large current necessary for sufficiently inverting spontaneous polarization to flow.
When an optically uniaxially orientation means is provided on a surface of substrates between which ferroelectric or antiferroelectric liquid-crystal material is interposed between the substrates with the orientation means being in direct contact with the liquid crystal, there has arisen a problem on the switching operation of the liquid-crystal molecules.
For example, in the case of using a rubbing method, in a process where the liquid-crystal material is gradually cooled after it has been injected into the cell, although the rubbing direction and the orienting vector have been arranged in parallel with each other in the SmA phase, the liquid-crystal molecules provide an inclined tilt angle with respect to a normal of the smectic layers when the SmA phase is transit to the SmC* phase. Therefore, the orienting vector of the liquid-crystal molecules is not arranged in parallel with the rubbing direction, as a result of which a bistable stable can be obtained. However, since the above-mentioned rubbing direction exists in an intermediate position between the two stable states of the liquid-crystal molecules, the liquid-crystal molecules are affected by the optically uniaxial orientation controlling force during the switching operation, thereby to obstruct the switching operation of the liquid-crystal molecules.
On the other hand, although the orienting method using a physical means such as the above-mentioned shearing method, the magnetic orienting method, the temperature gradation method is not so practical for mass production, because what produces the optically uniaxial orientation regulating force does not exist after the liquid-crystal material is oriented, the switching operation is not obstructed so that an excellent switching characteristic is obtained.
Therefore, in the ferroelectric liquid-crystal electro-optical device where an optically uniaxially orientation means is formed on a surface which is brought in contact with the liquid-crystal material to orient the liquid-crystal molecules, there have arisen such problems that the switching speed is lowered, the switching operation is insufficiently performed or the like, in comparison with the liquid-crystal electro-optical device where the liquid-crystal molecules are oriented by the shearing method, the magnetic orienting method, the temperature gradation method or the like.
Further, in the liquid-crystal electro-optical device using the ferroelectric liquid crystal or the antiferroelectric liquid crystal, more particularly, in the so-called surface stabilizing ferroelectric liquid crystal (SSFLC) having a substrate gap of several .mu.m and a structure for restraining the spiral structure of the liquid-crystal molecules, because it has the bistable property, the obtained light transmission state has only two bright and dark states, as a result of which the halftone obtained by using the nematic liquid crystal could not been obtained. That is, the amount of the transmitted light could not be continuously changed in accordance with the state change of the liquid-crystal molecules.
In the case where the polarity of an electric field is inverted to switch between a first state and a second state, the SSFLC device performs the switching operation between the above-mentioned two states if the liquid crystal is driven by the strength of the electric field more than a saturation voltage. However, when the strength of the electric field is gradually changed, switching is not performed by uniformly changing the amount of the transmitted light of the entire liquid-crystal material in a region to which the electric field is applied, but the following switching is usually performed. For example, when switching is made from the first state to the second state, a region where the first state is inverted into the second orienting state (hereinafter referred to as "domain") occurs in a region where the first state is exhibited. Under the condition, when the strength of the electric field is further increased, an area of the domain is enlarged thereby to move to the second state.
One of methods of obtaining a halftone in the liquid-crystal electro-optical device using the ferroelectric liquid crystal or the antiferroelectric liquid crystal, using the above-mentioned property, is the area gradation method.
Observing a process of inverting the liquid-crystal molecules of the ferroelectric liquid crystal or the antiferroelectric liquid crystal with a polarization microscope, voltage is applied so that the dark-state region occurs in the bright-state region or the bright-state region (hereinafter referred to as "domain") occurs in the dark-state region in a specified region to which voltage is applied. Further, when voltage is kept applied, the area of each domain is widened in such a manner that the bright state comes to the dark state or the dark state comes to the bright state as the whole specified region.
The area gradation method is to control the area of the bright or dark domain within one pixel by controlling applied voltage, using the fact that the dimensions of the domain are slightly changed with the change of the applied voltage, thereby to obtain a halftone.
Further, in the surface stabilizing type liquid-crystal electro-optical device using the ferroelectric liquid crystal or the antiferroelectric liquid crystal, there is a pixel dividing method as another method of obtaining a halftone. This method is to constitute one pixel by a plurality of small pixels so as to obtain a halftone by combination of two bright and dark states of the respective small pixels.
For example, when one pixel is constituted by four small pixels, the darkest or brightest state makes all of the four small pixels in the dark or bright state. Also, when a halftone is to be obtained, for example, one of four small pixels is kept in the dark state whereas three remaining pixels are kept in the bright state, whereby a halftone having the transmitted light amount of 75% is obtained as one pixel in comparison with the brightest state.
Thus, when a halftone is to be obtained in the liquid-crystal electro-optical device using the ferroelectric liquid-crystal or the antiferroelectric liquid crystal, it must rely on the above-mentioned area gradation method which controls the dimensions of the domain, the pixel dividing method which falsely represents one pixel by a plurality of small pixels, or the like.
However, in the area gradation method, because the inversion of the ferroelectric or antiferroelectric liquid crystal is rapid, in particular, the domain is largely expanded even though an applied voltage value is slightly changed, the width of voltage capable of realizing the area gradation is remarkably narrowed and it has a hysteresis. For this reason, it is difficult to control the domain area by the applied voltage of several mV unit. Also, when the applied voltage is lowered, a response speed is remarkably low, as a result of which uniform display is not enabled. Furthermore, it is difficult to elevate the resolution of display, and therefore this method has not been practical.
Further, in the pixel dividing method, the efficiency is low because a plurality of pixels are used for one pixel, and also there is technically a limit to increase the number of pixels by reducing the area of one pixel. Also, it is improper to make the resolution high.
Still further, in the conventional surface stabilizing type liquid crystal electro-optical device using a ferroelectric liquid crystal or an antiferroelectric liquid crystal, a low-voltage drive could not be expected because it has a high threshold value.
Further, in the conventional surface stabilizing type liquid crystal electro-optical device, the orienting process subjected to a pair of substrates is different between both the substrates so that the orienting stability of the liquid-crystal material is kept monostable, and the strength of the electric field is changed, as a result of which gradation display is performed. However, even in this method, consequently the inversion with the domain is made at the switching operation, and the same difficulty as the above-mentioned bistable type follows. Therefore, it is not sufficient as the gradation display.
Therefore, in the liquid-crystal electro-optical device using the ferroelectric or antiferroelectric liquid-crystal material, it has been desired to provide a structure which can perform an excellent halftone gradation display.
Further, when the frame gradation method is used in the active matrix type antiferroelectric liquid-crystal electro-optical device, in order to perform high-speed and high-contrast display, it is required that the inversion of the liquid-crystal molecules of the ferroelectric liquid crystal at the time of applying a signal is conducted in an extreme short time, the inversion is sufficient, and the state of the liquid-crystal molecules after conversion is kept constant without any changes. The state of the liquid-crystal molecules is mainly determined in accordance with the strength of voltage applied to the electrodes of both the substrates.
By the way, there has been well known that an impurity having charges transferred from the liquid crystal or the orientation film exists in the device, and there occur surplus charges which allow voltage to be generated in a direction reverse to applied voltage when applying voltage. These charges freely move within the liquid-crystal material which is held between both the substrates with application of voltage. A lot of these charges move to reach the surface of the orientation film, and because the orientation film is naturally insulative, the charges then move no more in such a manner that they are accommodated on a liquid-crystal interface of the orientation film between the orientation film and the liquid-crystal layer (a layer of the liquid-crystal material).
These charges cause a problem which is of disadvantage to the liquid-crystal electro-optical device. For example, action which cancels voltage applied between the electrodes is produced, resulting in decrease of contrast. For example, when pulsed voltage is applied by driving the TFT, there occurs such a two-steps response that the transmission or non-transmission switching is not rapid in such a manner that, after the transmission or the non-transmission is switched once, it is further switched with a small delay, or attenuation is generated immediately after switching operation. In order to solve this problem, it is required that applied voltage is made larger than voltage necessary for inverting the spontaneous polarization. However, this is not a satisfactory countermeasure.
When voltage is applied between the electrodes, the state of the liquid-crystal molecules is not stabilized because the charge amount within the liquid-crystal layer is changed as a time elapsed. Further, the liquid-crystal molecules which have been electrically attracted by the charges stored in an interface between the orientation film and the liquid-crystal has larger voltage required for change of the state than the liquid-crystal molecules within the liquid-crystal layer, which are not attracted. Therefore, the liquid-crystal molecules within the liquid-crystal layer are not changed in state together, but, the light transmission characteristic which is most important as the characteristics of the liquid-crystal electro-optical device is not stabilized.
Then, as the liquid-crystal electro-optical device, the display is unstable, the display speed is unavoidably lowered without making use of the high speed capability of the liquid crystal material, and the contrast is lowered. In particular, when performing frame gradation display, the number of gradations is limited.
In order to solve this problem, there is an attempt in which the orientation film material relaxing the storage of charges is selected, or a method in which SiO or the like is obliquely vapor-deposited on the electrodes instead of using the orientation film of an insulating film. However, in such methods, it takes time to conduct a large number of preliminary experiments, which causes the costs to rise, and also its effect is changed in accordance with combination of the materials used. Therefore, such methods are not general. Also, there is a method of removing an impurity by purifying the liquid-crystal. However, in this method, the purified liquid crystal which is usable is very slight so that it is very improper to mass-productivity. Further, there is a method in which charges existing within the liquid-crystal layer is absorbed or coupled by using charge transfer complex or the like in such a manner that positive or negative charges are canceled or neutralized. However, it is difficult to measure and insert the charge transfer complexes by the amount necessary for completely canceling charges within the device, as a result of which surplus charge transfer complexes move within the liquid-crystal layer, likewise as the above-mentioned charges.
As mentioned above, there have been proposed various methods for canceling a factor which causes change of voltage applied to the liquid-crystal layer, that is, charges existing within the liquid-crystal layer, which is a factor for changing the liquid-crystal molecules as a time elapsed to unstabilize the optical characteristics of the device, however, it is difficult to readily and completely cancel such a factor.